Regional cerebral blood flow and structural brain changes are being studied through a 5-year longitudinal protocol involving asymptomatic persons with and without the Huntington's disease mutation. This will document the earliest neuroimaging evidence of HD and the relation between neuroimaging changes and clinical course. We have enrolled 25 mutation-carriers and 29 non-carriers. First year follow-up scans have been completed on 13 carriers; second year follow-up scans have been completed on 3 carriers. Repeat scans have been completed on 5 non- carriers.